Vehicles may include steering systems that pivot steered wheels of the vehicle about generally vertical steering axes. Such steering systems may be operable by, e.g., rotating a steering wheel, which may be connected by a steering gear and mechanical linkage to the steered wheels. Some steering systems may include assistance systems, such as hydraulic or electrical power steering systems. These systems may reduce the force (e.g., torque) an operator must apply to a steering control, such as a steering wheel, to rotate the steered wheels of the vehicle about the steering axes. Such systems may include hydraulic and/or electrical components configured to apply a torque or force to the steering system of the vehicle to augment the operator's input and thereby reduce the steering effort.
Steering effort may be highest when the vehicle is stationary with the brakes depressed. Under these conditions, movement of the steered wheels about the steering axes may cause the portion of the steered wheels in contact (e.g., a tire contact patch) with a surface (e.g., road) on which the vehicle rests to “scrub” against the surface. In other words, a static frictional force between the vehicle tires and the road surface must be overcome to steer the wheels. This frictional force may be significant, particularly in heavy vehicles, vehicles with the engine and/or drivetrain positioned above the steered wheels, vehicles with large tire contact patches, etc. Components of steering assistance systems must be capable of producing enough force to overcome this frictional force. Furthermore, in the case of vehicles where no steering assistance is provided to the operator (i.e., manually steered vehicles), the force required to steer the wheels may be greater than the operator can comfortably provide, and/or the steering ratio (i.e., the mechanical advantage provided the operator by the steering mechanism) may be undesirably high.
It is desirable to provide systems capable of reducing the required steering force when the vehicle is at rest, to allow utilization of steering components of lesser power, weight, and cost, and/or to reduce the steering effort required to steer the wheels.